Officers freed two children who were shackled to beds and arrested the parents in a case that drew worldwide attention to severe neglect at the Perris, California, home that looked neatly kept from the outside.

Inside, police discovered something completely different.

The house reeked of human waste, and the evidence of starvation was obvious, with the oldest of 13 siblings weighing just 38kgs. The children were locked up as punishment, denied food and toys and allowed to do little except write in journals, prosecutors said.

Neighbours write down messages of support for the Turpin's children on the front door of the home of David and Louise Turpin. (AAP) ()

Children were isolated from each other and locked in different rooms in small groups. They did not have access to televisions or radios but expressed themselves in the hundreds of journals that investigators seized from the home where they were schooled.

After they were freed, the children, who ranged in age from 2 to 29, were immediately hospitalised and eventually released.

Their current whereabouts are unknown. A spokeswoman for the county's social services department declined to discuss the case.

Jack Osborn, a lawyer appointed to represent the seven adult children, said earlier this year they were "doing well." They have participated in music therapy programs, made crafts and world-famous cellist Yo-Yo Ma held a special concert for them. They communicated with their younger siblings over Skype.

"They're happy, they are wanting to move forward, they do not want to dwell on the past," he told the Riverside Press-Enterprise newspaper in February.

"They want their identity to be now and going forward the things they hope to do, the dreams they have. They do not want people to think of them only as a possible victim, but as young adults setting off on their lives."

Osborn did not respond to an email from The Associated Press this week.

Prosecutors planned to have sheriff's deputies testify at Wednesday's hearing, but none of the children was expected to take the stand.

David Turpin's lawyer, David Macher, said he was "looking forward to the hearing," but declined to comment further. Louise Turpin's lawyer did not immediately respond to request for comment.